Autosomal dominant polycystic kidney disease arises from mutations in either the PKD1 or the PKD2 gene which encode PC1 or 2. Although it is increasing clear that PC1 is expressed at the plasma membrane and the primary cilium, the cellular localization of PC2 has been controversial. PC2 localizes to the ER, the plasma membrane, and the primary cilium. A popular hypothesis is that PC1 acts as a mechanical receptor and PC2 as a Ca2+ influx channel in the primary cilium to sense renal flow. It has also been suggested that PC2 functions as an intracellular Ca2+ channel. In addition, evidence suggests that PC1 mutants can cause additional mis-localization of PC2 from the cell surface to the ER, which might also contribute even further to the pathogenesis of ADPKD. Therefore, it is important to investigate the role of the ER localized PC2 in regulating intracellular Ca2+ signaling. We propose that PC1 and 2 operate at the ER and at the plasma membrane in concert with the primary cilium to regulate cytosolic Ca2+. Our studies indicate that both PC1 &2 functionally interact with the IP3 receptor (IP3R) to modulate intracellular Ca2+ signaling. IP3 mediated intracellular Ca2+ signaling is one of the most important intracellular signaling pathways. We also show that a fragment of PC1 which we call PEC inhibits Ca2+ dependent Cl- currents in oocytes suggesting that it inhibits Ca2+ entry. The fragment coimmunoprecipitates with a component of the store operate Ca2+ entry pathway, STIM1. The overall hypothesis is that PC1 &2 have a precise role in setting the range of intracellular Ca2+ concentrations over which Ca2+ is released from IP3R and simultaneously inhibiting SOCE. We speculate that PC1 inhibition of store-operated Ca2+ entry is necessary to poise renal cells for Ca2+ influx via the primary cilium as a major route of Ca2+ entry. The proposal has three essential parts: Part I. How specifically do the polycystins modulate ER Ca2+ release? Part II. Do the polycystins regulate store-operated Ca2+ entry? Part III. What is the role of IP3R and store-operated calcium entry in tubule and cyst formation? The overall focus of the past 25 years has been on the mechanisms of transport in proximal and distal tubules. During the tenure of this current award, we made fundamental discoveries regarding how ions move through aquaporin water channels and how PC-1 &2 form a functional complex. We expanded the previous scope into the area of mouse models. The current application takes a new direction into the area of intracellular Ca2+ signaling. PUBLIC HEALTH RELEVANCE Autosomal dominant polycystic kidney disease, a major cause of kidney failure, arises from mutations in more that one gene. When the polycystic kidney genes function abnormally, intracellular calcium is not regulated properly. Thus, understanding normal and abnormal calcium regulation in health and disease, respectively, will ultimately lead to new ways of treating this serious disease.